Polyamide products such as nylon 6 and nylon 12 are utilized in a large amount as nylon fibers, films and engineering plastics in various fields. Used polyamide products are treated in landfills or incineration as waste materials. However, various methods for recycling polyamide products have been considered in recent years from the viewpoints of environmental conservation and effective utilization of resources. For example, Patent Literature 1 describes a method including depolymerizing a polyamide in water in the presence of a nitrogen-containing compound. Furthermore, Patent Literature 2 describes a method for depolymerizing an ε-caprolactam oligomer to ε-caprolactam, which includes contacting the ε-caprolactam oligomer with water at a high temperature and a high pressure at a reaction temperature of from 280 to 450° C. and a pressure of from 100 to 500 kg/cm2.
However, ε-caprolactam that is obtained by, for example, depolymerization of nylon 6, has little use other than utilization as a monomer for nylon 6. In view of chemical recycle, there is a problem that, when waste materials from nylon 6 are reused in various applications, the waste materials need to be decomposed into hydrogen, carbon monoxide, methane and the like, and a large amount of energy is consumed therefor.
On the other hand, hydroxycarboxylic acid derivatives such as 6-hydroxycaproic acid esters are useful as intermediate materials for general organic synthesis, and are specifically utilized in various applications such as cationic coagulants, intermediates for medicaments and agrochemicals, softening agents for synthetic fibers, anticorrosives and dispersing agents.
Therefore, the inventors have found a method for obtaining a 6-hydroxycaproic acid ester by acting an alcohol in a supercritical state on nylon 6 or the like, aiming at obtaining a hydroxycarboxylic acid derivative that can be reused in various applications from a polyamide without consuming a large amount of energy (Patent Literature 3).